Organophosphorus pesticides (OPs) are the most commonly used pesticides in the U.S. and worldwide. Evidence from human and animal studies clearly identifies neurotoxicity as the primary endpoint of concern. However, it has been difficult to predict the risk that repeated low-dose exposure to OPs pose to humans because: 1) a relationship between OP dose and neurobehavioral deficits has yet to be established in humans;2) biomarkers that reliably predict OP-induced neurobehavioral deficits are not available: and 3) the potential for genetic variation to modify exposure sensitivity has not been thoroughly investigated. The proposed studies will test the hypotheses that OP-induced neurobehavioral deficits are dose-related and that measures of oxidative stress and inflammation are better predictors of neurobehavioral deficit than cholinesterase inhibition. These hypotheses will be tested by studying a cohort of pesticide application workers in Egypt's Menoufia Governorate previously reported to exhibit the broadest range of neurobehavioral deficits in humans following OP exposure. This Egyptian cohort is uniquely suited for these studies because, unlike most pesticide exposures, the exposure is simple (a single OP, chlorpyrifos) and consistent within job categories, but with substantial differences between job categories. In aim 1, OP doses will be estimated using PBPK/PD modeling of urinary OP metabolite data collected from 255 Egyptian workers over the application cycle. These workers will also be genotyped for polymorphisms of key enzymes involved in OP metabolism (CYP2B6, CYP2C19 and PON1) to evaluate the potential for genetic variation to modify internal dose. In aim 2, behavioral deficits will be determined in a subset of workers exhibiting a range of OP exposures. Data from aims 1 and 2 will be integrated to determine the relationship between OP dose and neurobehavioral deficits. Rat studies will be conducted in parallel (aim 3) to test candidate biomarkers as predictors of OP-induced neurobehavioral deficits. The specific biomarkers that will be examined include cholinesterase inhibition, urinary isoprostanes as a measure of oxidative stress, and serum levels of C-reactive protein and inflammatory cytokines as measures of inflammation. In aim 4, those biomarkers that predict OP-induced neurobehavioral deficits in rats will be tested to determine if they similarly predict deficits in behavioral performance in Egyptian pesticide workers. The proposed studies will provide critical data needed to develop effective biomarkers of OP exposure, biological response and genetic susceptibility. The availability of such biomarkers would facilitate the identification of at-risk individuals as well as the testing of intervention and treatment strategies, and the need to develop these strategies is underscored by evidence of widespread human exposure to OPs and the credible threat of OPs as chemical agents of terrorism. Project Summary/Abstract - Relevance The goal of the proposed studies is to identify biomarkers of exposure and effect that are predictive of neurobehavioral deficits in humans exposed to organophosphorus pesticides (OPs). In addition, we will examine human genetic variants of the enzymes CYP2B6 and CYP2C19 that influence OP metabolism to not only inform interpretation of OP exposure data, but also provide insights into genetic susceptibilities that modulate neurotoxic responses to OPs. These studies will provide data critically needed to identify at-risk individuals and will provide tools to facilitate the development and evaluation of intervention and treatment strategies for exposure to not only OP pesticides, which are the most commonly used pesticides in the U.S. and worldwide, but also to nerve agents that are considered a credible terrorist threat.